Method of fabricating a semiconductor device

ABSTRACT

A method of fabricating a semiconductor device wherein leakage current of a capacitor is reduced is provided. The method comprises steps of forming a lower electrode of the surface of a semiconductor substrate, forming a silicon nitride film over the lower electrode, applying a first heat treatment whereby the silicon nitride film is annealed in an atmosphere containing oxygen, forming a dielectric film containing alkaline earth metals over the silicon nitride film, applying a second heat treatment whereby the electric film is annealed in an atmosphere containing oxygen, and forming an upper electrode on the surface of the dielectric film.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of fabricating asemiconductor device, and more particularly, to a method of forming acapacitor of the semiconductor device.

[0003] 2. Description of the Related Art

[0004] With an advance being made in miniaturization of a semiconductordevice, there has since arisen a need for further reducing a capacitorarea and so forth. A tantalum oxide film (hereinafter referred to as aTa₂O₅ film) having a high permittivity has recently attracted attentionas a dielectric film of a capacitor. The dielectric film needs to berendered thin for the capacitor to have a sufficient capacitance.However, if the dielectric film is formed too thin, this will causeoccurrence of leakage current therefrom. Deficiency of oxygen within theTa₂O₅ film, residual impurity carbon therein, reduction of the Ta₂O₅film due to oxygen being drawn into an electrode material, and so forthare regarded as causes for occurrence of leakage current from the Ta₂O₅film. The technique for performing oxygen annealing after formation ofthe Ta₂O₅ film as a method of making up for deficiency of oxygen thereinhas been disclosed in Japanese Patent Laid-Open H 9-121035, and JapanesePatent Laid-Open H 4-199828. And a method of preventing oxygen frombeing drawn into an electrode material by forming the electrode materialof a metal having a free energy greater than that for the Ta₂O₅ film hasbeen disclosed in Japanese Patent Publication H 6-82782.

[0005] However, with the technique for performing the oxygen annealingafter formation of the Ta₂O₅ film as disclosed in the technicalliterature described above, difficulty has been encountered in supplyingoxygen as far as the vicinity of the interface between the Ta₂O₅ filmand an underlayer thereof. Further, in the case of changing theelectrode material, there have been cases wherein the Ta₂O₅ film weresubjected to a damage caused by a fluorine-containing gas when formingan electrode over the Ta₂O₅ film.

SUMMARY OF THE INVENTION

[0006] In order to solve the problems described above, a typical methodof fabricating a semiconductor device according to the inventioncomprises steps of forming a lower electrode on the surface of asemiconductor substrate, forming a silicon nitride film over the lowerelectrode, applying a first heat treatment whereby the silicon nitridefilm is annealed in an atmosphere containing oxygen, forming adielectric film containing alkaline earth metals over the siliconnitride film, applying a second heat treatment whereby the dielectricfilm is annealed in an atmosphere containing oxygen, and forming anupper electrode on the surface of the dielectric film.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a view showing steps of a first embodiment of a methodof fabricating a semiconductor device according to the invention;

[0008]FIG. 2 is a view showing steps of a second embodiment of a methodof fabricating a semiconductor device according to the invention;

[0009]FIG. 3 is a flow sheet showing steps of a third embodiment of amethod of fabricating a semiconductor device according to the invention;

[0010]FIG. 4 is a flow sheet showing steps of a fourth embodiment of amethod of fabricating a semiconductor device according to the invention;and

[0011]FIG. 5 is a view showing leakage current from a capacitorfabricated by one of the embodiments of the method of fabricationaccording to the invention.

PREFERRED EMBODIMENTS OF THE INVENTION

[0012] [First Embodiment]

[0013]FIG. 1 is a view showing steps of a first embodiment of a methodof fabricating a capacitor region of a DRAM and the like according tothe invention. The first embodiment of the invention will be describedhereinafter with reference to FIG. 1.

[0014] By implanting ions into a p-type silicon substrate 11, a n-typediffused region 12 is formed therein. Subsequently, by use of chemicalvapor deposition (CVD) techniques, an oxide film 13 serving as aninterlayer dielectric is formed over the substrate to a thickness in therange of 100 to 1000 nm (FIG. 1-a).

[0015] By use of the photolithographic process and dry etching, acontact hole is formed in the oxide film 13 on top of the n-typediffused region 12. By use of the CVD method, polysilicon 14 doped withphosphorus atoms is embedded in the contact hole.

[0016] Then, a polysilicon layer highly doped at a dose on the order of1˜10×10²⁰ of phosphorus atoms/cm³ is formed throughout the surface ofthe oxide film 13, and the polysilicon layer is patterned in apredetermined shape by the well known photolithographic etchingtechniques, forming a lower electrode 15 of a capacitor (FIG. 1-b).

[0017] In the case wherein the lower electrode 15 is formed ofpolysilicon, embedding of the contact hole and formation of a lowerelectrode layer can be carried out by the one and same step ofprocessing.

[0018] By use of the CVD method, a silicon nitride film (Si₃N₄ film) 16is formed over the lower electrode 15 of the capacitor to a thickness of2 nm (FIG. 1-c).

[0019] In a stage where the silicon nitride film 16 has been formed, afirst annealing is performed in an oxygen atmosphere. The annealing ispreferably performed at a temperature in the range of 500 to 1000° C. bya rapid heating method. In carrying out this embodiment, the firstannealing was performed in the temperature range of 800 to 900° C.

[0020] As a result of the first annealing in the oxygen atmosphere,silicon atoms still unbonded within the Si₃N₄ film will be bonded withoxygen atoms. Since the first annealing is performed prior to formationof a Ta₂O₅ film, it is possible to solve a problem that oxygen will notbe supplied up to the vicinity of the interface between a Ta₂O₅ film andan underlayer thereof.

[0021] The Si₃N₄ film 16 is formed over the lower electrode 15 of thecapacitor. Even if the first annealing is performed in the oxygenatmosphere, the Si₃N₄ film 16 prevent a SiO₂ film having a very lowpermittivity from being formed on the surface of the lower electrode 15of the capacitor.

[0022] By use of the CVD method, a Ta₂O₅ film 17 is formed over theSi₃N₄ film 16 to a thickness of 13 nm. After formation of the Ta₂O₅ film17, a second annealing is performed in an oxygen atmosphere. The secondannealing is performed by a rapid heating method at a temperature lowerthan the temperature for the first annealing. In carrying out thisembodiment, the second annealing was performed at a temperature in theorder of 600 to 700° C., lower by 200° C. than for the first annealing(FIG. 1-d).

[0023] The second annealing is performed at a temperature lower thanthat for the first annealing. This prevents oxygen atoms from beingdrawn from the Ta₂O₅ film 17 into the Si₃N₄ film 16. The secondannealing may be performed in an ordinary heating furnace, however,annealing by the rapid heating method is preferable from the viewpointof preventing oxygen atoms from being drawn from the Ta₂O₅ film 17 intothe Si₃N₄ film 16.

[0024] Subsequently, by use of the CVD method, a TiN film is formed to athickness of 30 to 100 nm, and by the photolithographic etchingtechniques, an upper electrode 18 of the capacitor is formed, therebycompleting the capacitor (FIG. 1-e).

[0025] By use of this embodiment of the method of fabricating acapacitor according to the invention, an excellent capacitor with lessleakage current can be fabricated. FIG. 5 shows leakage current of thecapacitor fabricated by this embodiment of the method of fabricationaccording to the invention as compared with that of a capacitorfabricated by the conventional method of fabrication.

[0026] As shown in FIG. 5, the capacitor fabricated by the method offabrication according to the invention has leakage current less thanthat of the capacitor fabricated by the conventional method offabrication.

[0027] With this embodiment of the invention, the Si₃N₄ film 16 formedover the lower electrode 15 prevents oxidation of the lower electrode15. However, since the Si₃N₄ film 16 has a lower permittivity incomparison with the Ta₂O₅ film 17, it is desirable that the Si₃N₄ film16 is formed to a sufficiently thin thickness in comparison with athickness of the Ta₂O₅ film 17 in order to secure a large capacitance ofthe capacitor.

[0028] [Second Embodiment]

[0029]FIG. 2 is a view showing steps of a second embodiment of a methodof fabricating a capacitor region of a DRAM and the like according tothe invention. Parts corresponding to those previously described withreference to FIG. 1 are denoted by the same reference numerals. Thesecond embodiment of the invention will be described hereinafter withreference to FIG. 2.

[0030] The method of fabrication according to the second embodiment isthe same as that according to the first embodiment up to the steps ofperforming a second annealing after forming a Ta₂O₅ film 17 over a lowerelectrode 15 of a capacitor and a silicon nitride film 16 (FIG. 2-c).

[0031] Subsequently, by use of the CVD method, a TiN film is formed overthe Ta₂O₅ film 17 to a thickness of 1 to 10 nm after the step ofperforming the second annealing, and thereafter, a third annealing isperformed in an oxygen atmosphere. The third annealing is performed bythe rapid heating method at a temperature lower than that for the firstannealing described above.

[0032] As a result of the third annealing, an oxide film 21 of TiN forconstituting an upper electrode is formed over the Ta₂O₅ film 17 (FIG.2-d).

[0033] By use of the CVD method, a TiN film is formed over the oxidefilm 21 of TiN to a thickens of 10 to 20 nm so as to serve as an upperelectrode 18 of the capacitor, thus completing fabrication of thecapacitor (FIG. 2-e).

[0034] With this embodiment of the invention, the oxide film 21 of TiNfor composing the upper electrode is interposed between the Ta₂O₅ film17 and the TiN film serving as the upper electrode 18. Hence, it ispossible to prevent oxygen from being drawn from within the Ta₂O₅ film17 into the TiN film 18. Consequently, with this embodiment, leakagecurrent can be further reduced than in the case of the first embodiment.

[0035] [Third Embodiment]

[0036]FIG. 3 is a flow sheet showing steps of a third embodiment of amethod of fabricating a capacitor region of a DRAM and the likeaccording to the invention. The third embodiment of the invention willbe described hereinafter with reference to FIG. 3.

[0037] The method of fabrication according to the third embodiment isthe same as that according to the first embodiment up to the steps offorming a Ta₂O₅ film over a lower electrode of a capacitor.

[0038] With this embodiment, a second annealing is performed at atemperature in the range of 400 to 600° C. in a reducing atmosphere(containing hydrogen H₂, ammonia NH₃, and so forth) after the Ta₂O₅ filmis formed.

[0039] As a hydrogen atom has a atomic radius smaller than that of anoxygen atom, the hydrogen atom is susceptible to diffusion within theTa₂O₅ film. The hydrogen atoms diffused in the Ta₂O₅ film are bondedwith residual carbon contained in the Ta₂O₅ film. The residual carboncombined with the hydrogen atoms makes up a volatile matter such as CH₄etc. and undergoes volatilization. As a result of the second annealingdescribed, the residual carbon in the Ta₂O₅ film can be effectivelyremoved.

[0040] Subsequently, a third annealing is performed in an oxygenatmosphere. The third annealing is performed by the rapid heating methodat a temperature lower by 200° C. than that for a first annealingpreviously performed.

[0041] Thereafter, by use of the CVD method, an upper electrode of thecapacitor is formed so as to serve as the upper electrode of thecapacitor, thus completing fabrication of the capacitor.

[0042] With this embodiment of the invention, the second annealing isapplied to the Ta₂O₅ film in the reducing atmosphere (containinghydrogen H₂, ammonia NH₃, and so forth), thereby reducing the residualcarbon. Hence, it is possible to further reduce leakage current than inthe case of the first embodiment of the invention.

[0043] [Fourth Embodiment]

[0044]FIG. 4 is a flow sheet showing steps of a fourth embodiment of amethod of fabricating a capacitor region of a DRAM and the likeaccording to the invention. The fourth embodiment of the invention willbe described hereinafter with reference to FIG. 4.

[0045] The method of fabrication according to the fourth embodiment isthe same as that according to the first embodiment up to the steps offorming a Ta₂O₅ film over a lower electrode of a capacitor.

[0046] With this embodiment of the invention, during a period subsequentto formation of the Ta₂O₅ film, oxygen atoms are implanted into theTa₂O₅ film by means of ion implantation at implantation energies rangingfrom 10 KeV to 1 MeV. After the oxygen atoms are implanted therein, anannealing treatment is applied at a temperature in the range of 500 to1000° C. in order to cause the oxygen atoms to be diffused.

[0047] Thereafter, by use of the CVD method or the equivalent, an upperelectrode of the capacitor is formed so as to serve as the upperelectrode of the capacitor, thus completing fabrication of thecapacitor.

[0048] With this embodiment of the invention, oxygen atoms are suppliedto the Ta₂O₅ film by the ion implantation method instead of by theannealing in an oxygen atmosphere. Hence, the oxygen atoms can besupplied thereto more effectively and with better control than in thecase of the annealing in an oxygen atmosphere.

[0049] [Fifth Embodiment]

[0050] Next, a fifth embodiment of a method of fabricating a capacitoraccording to the invention will be described. With the fifth embodimentof the invention, steps of a method of fabricating the capacitor are thesame as those described with reference to the first embodiment.

[0051] In this embodiment, a lower electrode 15 formed at first is notmade of polysilicon, but made of TiN, the same kind of material that anupper electrode to be formed later is made of.

[0052] That is, after the polysilicon 14 is embedded as shown in FIG. 1,the lower electrode 15 is formed by use of the CVD method or theequivalent.

[0053] A difference in work function between the lower electrode and anupper electrode of a capacitor due to a difference in material used isone of causes for occurrence of leakage current.

[0054] Accordingly, with this embodiment of the invention, both thelower electrode and the upper electrode are made of the same material,thereby enabling further reduction in the leakage current to beachieved.

[0055] Further, with reference to various embodiments described in theforegoing, use of the Ta₂O₅ film as a dielectric film has beendescribed, however, similar effects can be obtained by use of adielectric film other than the Ta₂O₅ film, containing alkaline earthmetals such as (Ba,Sr)TiO₃, Pb (Zr, Ti)O₃, and so forth.

What is claimed is:
 1. A method of fabricating a semiconductor devicecomprising steps of: forming a lower electrode on the surface of asemiconductor substrate; forming a silicon nitride film over the lowerelectrode; applying a first heat treatment whereby the silicon nitridefilm is annealed in an atmosphere containing oxygen; forming adielectric film over the silicon nitride film; applying a second heattreatment whereby the dielectric film is annealed in an atmospherecontaining oxygen; and forming an upper electrode on the surface of thedielectric film.
 2. A method of fabricating a semiconductor deviceaccording to claim 1, wherein the second heat treatment is applied at atemperature lower than that for the first heat treatment.
 3. A method offabricating a semiconductor device comprising steps of: forming a lowerelectrode on the surface of a semiconductor substrate; forming a siliconnitride film over the lower electrode; applying a first heat treatmentwhereby the silicon nitride film is annealed in an atmosphere containingoxygen; forming a dielectric film containing alkaline earth metals overthe silicon nitride film; applying a second heat treatment whereby thedielectric film is annealed in an atmosphere containing oxygen; formingan oxide film over the dielectric film; and forming an upper electrodeon the surface of the oxide film.
 4. A method of fabricating asemiconductor device according to claim 3, wherein the second heattreatment is applied at a temperature lower than that for the first heattreatment.
 5. A method of fabricating a semiconductor device accordingto claim 3, wherein the oxide film formed over the dielectric film isthe same kind of oxide film that is used for material of the upperelectrode.
 6. A method of fabricating a semiconductor device comprisingsteps of: forming a lower electrode on the surface of a semiconductorsubstrate; forming a silicon nitride film over the lower electrode;applying a first heat treatment whereby the silicon nitride film isannealed in an atmosphere containing oxygen; forming a dielectric filmcontaining alkaline earth metals over the silicon nitride film; applyinga second heat treatment whereby the dielectric film is annealed in areducing atmosphere; applying a third heat treatment whereby thedielectric film is annealed in an atmosphere containing oxygen; andforming an upper electrode on the surface of the dielectric film.
 7. Amethod of fabricating a semiconductor device comprising steps of:forming a lower electrode on the surface of a semiconductor substrate;forming a silicon nitride film over the lower electrode; applying a heattreatment whereby the silicon nitride film is annealed in an atmospherecontaining oxygen; forming a dielectric film containing alkaline earthmetals over the silicon nitride film; implanting oxygen ions into thedielectric film; and forming an upper electrode on the surface of thedielectric film.